Can a Nuclear Plant Shutdown Safely After an EMP?

If some unfriendly nation decides to hit America with an EMP attack there really won’t be a lot in the way of good news. If there is one bright spot in the whole mess it’s that EMP weapons explode way above the atmosphere, so none of their physical effects reach the ground. We might lose all our electronics and be knocked back to the 19^th century in the space of a minute, but at least we won’t have blast, thermal pulse and radioactive fallout to deal with.

Or will we?

An EMP warhead won’t directly expose us to nuclear effects, but it could do it indirectly. One of the most chilling things about EMP is that it can turn our own technology against us, turning things we rely on into deadly threats.

Despite a few well-publicized accidents nuclear power is one of the safest ways of generating electricity ever discovered. The total death toll from nuclear accidents is lower than the number of people who die in coal mine accidents every year (mostly in China). It’s dwarfed by how many died when the Möhne dam was destroyed in 1943. Nuclear reactors are scary – as are most big, complicated things that get very hot – but you really aren’t very likely to be harmed by one.

The problem is, if there’s an EMP attack that could change in a hurry. Right now there are 99 operational nuclear power stations in the USA, producing a fifth of the nation’s electricity. An EMP could trigger a series of catastrophic failures, by taking away the power the reactors need to operate safely.

Selling Ice to the Eskimos?

You’re probably thinking “What?” right now. It does sound bizarre. These stations produce electricity – huge amounts of it – so why would they suddenly run out of power? The answer is slightly complicated, and potentially very frightening.

Because of the damage they can cause if they go wrong, nuclear reactors are built with “fail safe” as one of the basic principles. At any moment, a whole range of systems are actively working to stop the reactor from shutting down. If a major component fails, a bomb goes off in the control room or an EMP detonates a few hundred miles above it, those systems stop working and the reactor rapidly starts to shut down.

The key step is the insertion of the control rods into the reactor core. These rods are made of materials that absorb neutrons, like boron and silver. It’s neutrons that make the chain reaction in a reactor happen; when atoms of plutonium or uranium decay, they release neutrons. Some of these hit other atoms, splitting them – and releasing more neutrons. Reactor fuel isn’t enriched enough for the reaction to reach explosive speeds, so all that happens is a lot of heat is produced and soaked up by the coolant. The coolant then passes it on through heat exchangers, boiling water into steam that spins the generator turbines.

If you can absorb enough of the neutrons, instead of letting them hit other atoms, the chain reaction will stop. That’s what the control rods are for. By moving them in and out of the reactor core the operators can control the speed of the reaction and regulate how much energy is released, or they can shut it down altogether by inserting the rods the whole way.

Because reactors are designed to fail safe, all the control systems are built to keep the rods out of the core. If the systems fail the rods will simply fall into the fully inserted position, the neutron flux will be cut to below operating levels, and the chain reaction will stop. Within seconds of a major EMP attack, every reactor in the USA will have its rods in place and be shutting down. So, problem solved?

Will Reactors Really Fail Safe?

No, unfortunately not. The problem is that the vast amount of heat produced by splitting atoms doesn’t just go away when the reaction stops. It takes a long time for the reactor to cool down below the coolant’s boiling point. That doesn’t matter as long as the coolant is still circulating through the heat exchangers – but keeping the coolant moving takes power, and the reactor has just shut down.

If the coolant pumps stop, what happens is the coolant in the actual reactor vessel just keeps soaking up the residual heat. It’s under high pressure, to raise its boiling point, but eventually it will boil anyway. When it does, the pressure will blow the cooling system – and probably the whole reactor vessel – apart. With the cooling system gone the remaining fuel will melt down, possibly burning its way through the bottom of the reactor vessel, but the real problem is the explosion itself.

When a reactor overheats and explodes, the escaping steam will be highly radioactive. The coolant water is radioactive anyway, because it’s been flowing through the reactor vessel. The force of an explosion will also scatter debris from the reactor; some of this can be huge chunks weighing tons, but there will also be a lot of pulverized concrete, much of it radioactive, and that can be thrown high into the air by the blast. Some of it will even be small enough to attach itself to water droplets in the escaping steam, and once that happens the winds can carry it for thousands of miles.

Meanwhile, the heat generated by the meltdown will start an intense fire that’s likely to quickly engulf the whole reactor. Temperatures in a melting core can reach 9,000°F, so pretty much everything around it burns. It can take days to contain it, never mind put it out – and until it’s contained it will be belching out a plume of radioactive smoke. When the Chernobyl plant blew up following a bungled safety test in 1986, the fire in Number 4 reactor wasn’t contained for nine days. Hundreds of millions of people in the USSR and western Europe were in the path of the radioactive smoke plume; traces of radiation made it all the way around the world.

Related: What To Pack In Case of A Nuclear Attack

The Critical Weakness

Our reactors are better designed and built – mostly – but the effects of an EMP would turn them into 99 potential Chernobyls. If the power grid is taken out, all our reactors will instantly go into shutdown mode – and if their coolant pumps stop running, they’ll almost certainly melt down. To stop that happening all nuclear power plants have their own backup generators. These power the coolant pumps for both the reactors and the spent fuel cooling ponds, which can also melt down if they aren’t cooled. It’s an effective system, but what happens if the backup generators fail too?

In fact, we already know the answer to that one: Fukushima happens. The initial earthquake that hit Fukushima put the reactors into a safe shutdown mode; then the tsunami took out the generators that were essential for the shutdown to stay safe. If an EMP takes out the generators, exactly the same will happen at every one of the USA’s 99 power stations as happened at Fukushima.

The unanswered question is what effect an EMP will have on the backup generators. In fact a lot of the generators themselves will probably be unaffected; they’re just big Diesel engines, and those can be pretty simple. Older Diesels don’t need any electronics to run, so as long as they can get started, they’ll work.

Where it gets difficult is starting them. These backup systems kick in automatically when the reactor shuts down, but that process does need electronics. Unless the reactor’s systems tell the generator to start, it won’t. In some cases the station’s operators might be able to start the generators manually and keep the pumps running, but that’s going to depend on the generators. If they have electronic control systems, instead of plain old fuel injection, they’re going to fail.

It’s likely that, even in the chaos that would follow an EMP attack, many reactors would still manage to shut down safely. However, plenty more probably won’t. The smart assumption to make is that at least half of them will have some sort of failure, and many will be as bad as Chernobyl or Fukushima.

Unfortunately, most of the USA’s nuclear reactors are on or near the east coast, and that means the prevailing winds will carry the steam and smoke inland. Large parts of the Midwest will be exposed to fallout. It won’t be anything like the fallout from a nuclear attack – a reactor accident will scatter a few tons of radioactive dust, while a nuclear ground burst will throw a few thousand tons into the atmosphere – but it could be bad enough.

Worst of all, with the media wiped out you probably won’t know what’s happening beyond the range of any radios you’ve managed to keep working. That means you have no idea if a reactor in the next state has turned itself into a radioactive bonfire. So if the power suddenly goes out and you think an EMP attack has happened, react the same way as you would if it was a nuke; prepare a fallout room as fast as you can, and stay under cover for two weeks or until you get more information.

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